Generally, polishing a surface of an article is conducted by a mechanical polishing method in which the surface is abraded by a mechanical force or a physical force, by an electropolishing method in which the surface is electrochemically eroded.
In a representative mechanical polishing, a surface of an article is abraded by an abrasive stone. Such a method can not be applied to polishing an inner face of a hole having a small diameter so that an abrasive stone can not get into the hole. Moreover, there may be left scratches by crystalline particles in the abrasive stone, thermal strain due to high temperature during abrasion, and debris on the face after finishing to polish.
Although the electropolishing method does not involve problems related to scratches and thermal strain, bubbles generated during electrochemical reaction may become attached to the face to be polished to hinder electrolysis, whereby pits are left on the face after finishing to a polish. This problem is serious in an inner face of a deep hole. Moreover, in the case that the face to be polished is coated with the resultant from electrolysis, reaction rate is lowered, whereby the time taken to polish lengthens.